Perioperative 3D transoesophageal echocardiography. Part 1: fundamental principles

Learning objectivesBy reading this article, you should be able to:•Discuss the technological progress that facilitated the development of perioperative three-dimensional transoesophageal echocardiography (3D-TOE).•Illustrate the technical principles that govern the process of 3D image generation in echocardiography.•Describe the technical limitations of 3D-TOE.•Anticipate the future developments of 3D echocardiography and their clinical impact.Key points•Three-dimensional transoesophageal echocardiography (3D-TOE) is a mature technology with multiple perioperative applications.•Three-dimensional imaging uses volume-based rather than sector-based scanning (i.e. two-dimensional imaging).•Three-dimensional image generation relies on the acquisition of a dataset made of multiple two-dimensional ‘slices’ of a predefined volume by using matrix array transducers.•Processing of the dataset is a critical multistep process that allows the analytical interrogation and the display of the structure of interest in 3D. By reading this article, you should be able to:•Discuss the technological progress that facilitated the development of perioperative three-dimensional transoesophageal echocardiography (3D-TOE).•Illustrate the technical principles that govern the process of 3D image generation in echocardiography.•Describe the technical limitations of 3D-TOE.•Anticipate the future developments of 3D echocardiography and their clinical impact. •Three-dimensional transoesophageal echocardiography (3D-TOE) is a mature technology with multiple perioperative applications.•Three-dimensional imaging uses volume-based rather than sector-based scanning (i.e. two-dimensional imaging).•Three-dimensional image generation relies on the acquisition of a dataset made of multiple two-dimensional ‘slices’ of a predefined volume by using matrix array transducers.•Processing of the dataset is a critical multistep process that allows the analytical interrogation and the display of the structure of interest in 3D. Since its introduction into clinical practice by Matsumoto, over 40 yrs ago, transoesophageal echocardiography (TOE) has advanced constantly, becoming an essential part of perioperative care.1Matsumoto M. Oka Y. Strom J. et al.Application of transesophageal echocardiography to continuous intraoperative monitoring of left ventricular performance.Am J Cardiol. 1980; 46: 95-105Google Scholar Currently, TOE has a role in all open-heart, elective coronary artery bypass graft (CABG) and thoracic aorta surgical procedures.2American Society of Anesthesiologists, Society of Cardiovascular Anesthesiologists Task Force on Transesophageal EPractice guidelines for perioperative transesophageal echocardiography. An updated report by the American Society of Anesthesiologists and the Society of Cardiovascular Anesthesiologists task Force on Transesophageal Echocardiography.Anesthesiology. 2010; 112: 1084-1096Google Scholar, 3Nicoara A. Skubas N. Ad N. et al.Guidelines for the use of transesophageal echocardiography to assist with surgical decision-making in the operating room: a surgery-based approach: from the American Society of Echocardiography in collaboration with the Society of Cardiovascular Anesthesiologists and the Society of Thoracic Surgeons.J Am Soc Echocardiogr. 2020; 33: 692-734Google Scholar, 4Metkus T.S. Thibault D. Grant M.C. et al.Transesophageal echocardiography in patients undergoing coronary artery bypass graft surgery.J Am Coll Cardiol. 2021; 78: 112-122Google Scholar Three-dimensional (3D) TOE has developed and matured as a technology over the past 30 yrs, offering high-quality real-time (RT) imaging. The contemporary technology has become easier to use often with single-button acquisition and streamlined workflows. It can present large datasets with better temporal resolution and is capable of advanced on-line multiplane display. In addition, quantitative assessment of cardiac structures is possible through semi-automated parametric models. Clinically, 3D-TOE contributes actively to perioperative monitoring and diagnosis, informing surgical decision-making and detecting complications with proven utility in interventional cardiology and cardiac surgery. This article aims to provide an up-to-date review of the basic science of 3D-TOE imaging highlighting the technical requirements, limitations, challenges and future developments. In a forthcoming accompanying article we address the clinical applications of 3D-TOE. The term ‘3D’ characterises both still images and video clips, although from a physics concept, the latter represents 4D technology with the fourth dimension representing time. The idea of 3D echocardiographic imaging originated in the 1960s, but a technological lag delayed any meaningful results until 1974, when Dekker developed the first transthoracic echocardiography (TTE) 3D system.5Dekker D.L. Piziali R.L. Dong Jr., E. A system for ultrasonically imaging the human heart in three dimensions.Comput Biomed Res. 1974; 7: 544-553Google Scholar A mechanical articulated arm capable of measuring spatial probe displacement recorded multiple 2D images by free-hand movement. Historically, 3D image generation used off-line post-processing reconstruction techniques of 2D images obtained by either linear sequential multiplane scanning or gated (ECG or respiratory) acquisition of different cut-planes (Supplementary Fig. S1).6Mor-Avi V. Mumm B. Lang R.M. The evolution of three-dimensional echocardiography: from the initial concept to real-time imaging.in: Badano L. Lang R.M. Muraru D. Textbook of Three-Dimensional Echocardiography. Springer Nature, Cham, Switzerland2019Google Scholar Here, gating refers to ECG (i.e. R–R interval) or respiratory cycle monitoring with image acquisition during certain moments of the cycle, allowing for consistency (i.e. recording of a structure from multiple angles/position, at the same cycle phase over sequential cycles) and lack of motion artefacts. The raw 2D images were obtained by using either TTE freehand scanning, TTE/TOE mechanically driven transducers or multiplane TOE probes (capable of electronically rotated 2D imaging acquisition over a 180° arc).7Hung J. Lang R. Flachskampf F. et al.3D echocardiography: a review of the current status and future directions.J Am Soc Echocardiogr. 2007; 20: 213-233Google Scholar In 1991, there was sequential linear acquisition in a TEE probe, called the ‘lobster tail’, where a motorised mechanism in the probe handle activated a linear array of crystals, recording multiple parallel 2D ECG/respiration gated sectors at 1 mm increments.8Muraru D. Badano L.P. Physical and technical aspects and overview of 3D- echocardiography.in: Rojo E.C. Fernandez-Golfin C. Zamorano J.L. Manual of 3D Echocardiography. Springer International Publishing, Cham, Switzerland2017Google Scholar A TEE probe using rotational scanning was an alternative proposed 2 yrs later. The probe remained in a fixed position while the imaging plane rotated by pivoting around a fixed axis similar to the transducer of modern TOE probes.8Muraru D. Badano L.P. Physical and technical aspects and overview of 3D- echocardiography.in: Rojo E.C. Fernandez-Golfin C. Zamorano J.L. Manual of 3D Echocardiography. Springer International Publishing, Cham, Switzerland2017Google Scholar Motion artefacts, gating adequacy and the number of 2D images affected the 3D image quality. The next developments in RT3D ultrasound (US) systems occurred 20 yrs later.7Hung J. Lang R. Flachskampf F. et al.3D echocardiography: a review of the current status and future directions.J Am Soc Echocardiogr. 2007; 20: 213-233Google Scholar,9Sheikh K. Smith S.W. von Ramm O. et al.Real-time, three-dimensional echocardiography: feasibility and initial use.Echocardiography. 1991; 8: 119-125Google Scholar The fundamental challenge was the physical limitation of the fixed speed of sound in tissues. Overcoming this required the development of a transducer capable of volumetric acquisition and computing capacity to process a large amount of data, to present in an understandable 3D format for timely interpretation. This obliged engineering improvements in computational power and US probe design and technology. Von Ramm pioneered RT3D acquisition in the early 1990s by developing a matrix TTE transducer with 256 non-simultaneous firing (sparse) piezoelectric crystals capable of acquiring a 60°×60° pyramid volume in one heartbeat.7Hung J. Lang R. Flachskampf F. et al.3D echocardiography: a review of the current status and future directions.J Am Soc Echocardiogr. 2007; 20: 213-233Google Scholar,9Sheikh K. Smith S.W. von Ramm O. et al.Real-time, three-dimensional echocardiography: feasibility and initial use.Echocardiography. 1991; 8: 119-125Google Scholar These first-generation RT3D probes had poor spatial and temporal resolution, with image display of 2D cut planes from the original 3D dataset rather than a volume rendered 3D image. In 2006, the first RT3D-TOE probe became available.8Muraru D. Badano L.P. Physical and technical aspects and overview of 3D- echocardiography.in: Rojo E.C. Fernandez-Golfin C. Zamorano J.L. Manual of 3D Echocardiography. Springer International Publishing, Cham, Switzerland2017Google Scholar Contemporary RT3D-TOE systems have benefited from the technological advances that enabled miniaturisation of transducer crystals and circuitry. The availability of more powerful central processing units (CPUs), graphic processing units, digital signal processors and other essential components fuelled the transition from hardware to software-based architectures. A contemporary US machine generates RT3D images by relying on a powerful computer with multicore CPU architecture capable of larger memory and processing speed, which backs up a complex software package that performs tasks historically executed by analogue hardware. The current 3D-TOE systems can achieve volume rates two to three times higher (i.e. >60 Hz when using multi-beat acquisition) than their predecessors, with significantly improved spatial resolution.10Handke M. Heinrichs G. Moser U. et al.Transesophageal real-time three-dimensional echocardiography methods and initial in vitro and human in vivo studies.J Am Coll Cardiol. 2006; 48: 2070-2076Google Scholar,11Vegas A. Meineri M. Jerath A. Real-time three dimensional transesophageal echocardiography A step-by-step guide. Springer Science+Business Media; New York, USA, 2012Google Scholar The major difference between 2D and 3D-TOE imaging is that the latter uses volume-based rather than sector-based scanning. In 2D sector-based scanning, the transducer contains up to 128 piezoelectric crystals arranged in a single row, capable of acquiring and displaying one 2D image (slice) at a time. The modern 3D transducers have a similar footprint with a matrix design (i.e. x-y distribution) containing >2500–3000 fully sampled crystals, capable of generating a sound wave that is steerable in multiple directions (Fig. 1).13Lang R.M. Badano L.P. Tsang W. et al.EAE/ASE recommendations for image acquisition and display using three-dimensional echocardiography.Eur Heart J Cardiovasc Imaging. 2012; 13: 1-46Google Scholar, 14Vegas A. 3D TEE imaging.in: Perrino A.C. Reeves S.T. A Practical Approach to Transesophageal Echochardiography. 4th Edn. Wolters Kluwer, Philadelphia2020: 545-559Google Scholar, 15Rabben S.I. Technical principles of transthoracic three-dimensional echocardiography.in: Badano L.P. Lang R.M. Zamorano J.L. Textbook of Real-Time Three Dimensional Echocardiography. Springer-Verlag, London2011Google Scholar Beamforming is a signal processing technique where electrical signals are manipulated to generate directionally/spatially oriented US beams by the piezoelectric crystals. In addition, the beamformer handles the electrical signals (and their transmission to the US system) occurring in the crystals as a result of the returning echoes, also.16Muraru D. Badano L.P. Physics and technical principles of three-dimensional echocardiography.in: Badano R.M. Lang, L.P. Muraru D. Textbook of Three-Dimensional Echocardiography. Springer Nature, Cham, Switzerland2019Google Scholar In modern 3D-TOE systems, beamforming occurs both in the probe as fine (micro) beamforming and on the US machine computer system as coarse beamforming. This approach ensures a small probe and thin cable with less power consumption.15Rabben S.I. Technical principles of transthoracic three-dimensional echocardiography.in: Badano L.P. Lang R.M. Zamorano J.L. Textbook of Real-Time Three Dimensional Echocardiography. Springer-Verlag, London2011Google Scholar There is a complex proprietary algorithm for each vendor that processes the emitting and the returning US signal. In addition, by activating certain of crystals, is possible to have a display of multiple 2D images in (i.e. for two images and for three (Fig. 3D-TOE probes at and can all the of a probe, and interrogation and imaging relies on emitting through by recording and processing the returning The is to the and of 3D image processing and image display. of the returning raw US signal on and a 3D dataset of the sampled technology all 3D datasets as RT3D images that on the display as the probe The 3D dataset should have a spatial resolution to and temporal resolution for in a large although in as in the resolution for 3D is the volume in although often on the display. The spatial of and temporal resolution are to the 3D volume Three-dimensional datasets are as with the at the probe, and the The speed of sound in is a major challenge for 3D imaging. this is for the same a 3D volume more scanning than a 2D of the same in a volume scanning a volume of 60°×60° with a of achieve a volume of S.I. Technical principles of transthoracic three-dimensional echocardiography.in: Badano L.P. Lang R.M. Zamorano J.L. Textbook of Real-Time Three Dimensional Echocardiography. Springer-Verlag, London2011Google Scholar In at a 2D image with can achieve a Hz A. 3D TEE imaging.in: Perrino A.C. Reeves S.T. A Practical Approach to Transesophageal Echochardiography. 4th Edn. Wolters Kluwer, Philadelphia2020: 545-559Google Scholar the of the pyramid temporal a US to the of the 3D volume the next one can be a the for this in a of the is a of Hz can be the a certain and of the image the is S.I. Technical principles of transthoracic three-dimensional echocardiography.in: Badano L.P. Lang R.M. Zamorano J.L. Textbook of Real-Time Three Dimensional Echocardiography. Springer-Verlag, London2011Google Scholar the US system the when the is a certain S.I. Technical principles of transthoracic three-dimensional echocardiography.in: Badano L.P. Lang R.M. Zamorano J.L. Textbook of Real-Time Three Dimensional Echocardiography. Springer-Verlag, London2011Google Scholar A is essential for in the 3D US systems use as parallel beamforming and as multi-beat acquisition (Supplementary Fig. and volume to the volume S.I. Technical principles of transthoracic three-dimensional echocardiography.in: Badano L.P. Lang R.M. Zamorano J.L. Textbook of Real-Time Three Dimensional Echocardiography. Springer-Verlag, London2011Google Scholar beamforming (Supplementary Fig. the volume by one US by using multiple crystals to the sound wave in This the volume by a to the number of D. Badano L.P. Physics and technical principles of three-dimensional echocardiography.in: Badano R.M. Lang, L.P. Muraru D. Textbook of Three-Dimensional Echocardiography. Springer Nature, Cham, Switzerland2019Google Scholar This technique has as the in the the spatial D. Badano L.P. Physics and technical principles of three-dimensional echocardiography.in: Badano R.M. Lang, L.P. Muraru D. 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